Gas-phase polymerization processes are economical processes for producing polymers. Such processes can, in particular, be carried out in gas-phase fluidized-bed reactors, in which the polymer particles are kept in a fluidized state by means of an appropriate gas stream. To operate a gas-phase fluidized-bed reactor for the production of polyolefins, a very stable temperature control is critical. Otherwise oscillations in the reactor temperature could influence the polymerization behavior of the catalyst and not only eventually result in off-spec production but could also bring about difficulties in operating the reactor like, for example, lump formation or fouling in the reactor, or even shut-down of the reactor. Accordingly, to allow stable polymerization the reactor temperature should be kept within a narrow temperature window. Such a narrow temperature window is especially important for catalyst systems, which show high temperature sensitivity in their polymerization behavior like chromium-based catalysts or mixed catalyst systems.
A typical reactor set-up for a gas-phase fluidized-bed reactor for the production of polyolefins provides a closed loop for the reactor gas, in which the gas is fed in at the lower end of the reactor and taken off again at the upper end of the reactor. The withdrawn reactor gas is passed through a heat exchanger and a compressor and then recirculated to the reactor. The velocity of the reactor gas has to be sufficiently high, firstly to fluidize the bed of polymer particles and secondly to remove the heat of polymerization effectively.
Thus, to be able to react promptly to spontaneous or intended temperature changes in the reactor, it is needed to have a fast working control method for adjusting the temperature of the circulating gas. EP 516 036 A2 discloses a regulation method in which not the full amount of the circulating gas is passed through the heat exchanger but only a part of it. The ratio of the part which bypasses the heat exchanger and the part which flows through the heat exchanger can be controlled in response to the temperature in the reactor. Furthermore, also the temperature of the cooling medium can be controlled by partly recirculating the heated cooling medium leaving the heat exchanger back to the heat exchanger. For improving the response time of the cooling system, WO 00/00875 also suggests a method for controlling the temperature of the polymerization process by adjusting the temperature of the cooling medium entering the heat exchanger.
Although such cooling systems allow operating a gas-phase fluidized-bed reactor for the production of polyolefins, there is still a desire to improve the stability of the temperature in the reactor.
Thus, it was the object of the present invention to overcome the disadvantages of the prior art and to find a cooling method for the cycle gas which works with a shorter response time and a high accuracy in maintaining the reactor temperature. Accordingly the method allows carrying out the polymerization at a temperature very close to the melting point of the produced polyolefins if desired.